LM2672 datasheet(19/25 Pages) NSC

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LM2672 Series Buck Regulator Design Procedure (Adjustable Output)

(Continued)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

B. Select an appropriate capacitor value and voltage rating,

using the capacitor code, from the output capacitor selection

table in Figure 17. There are two solid tantalum (surface mount)

capacitor manufacturers and four electrolytic (through hole)

capacitor manufacturers to choose from. It is recommended that

both the manufacturers and the manufacturer’s series that are

listed in the table be used. A table listing the manufacturers’

phone numbers is located in Figure 11.

B. From the output capacitor selection table in Figure 17,

choose a capacitor value (and voltage rating) that intersects the

capacitor code(s) selected in section A, C20.

The capacitance and voltage rating values corresponding to the

capacitor code C20 are the:

Surface Mount:

33 µF/25V

Sprague 594D Series.

33 µF/25V

AVX TPS Series.

Through Hole:

33 µF/25V

Sanyo OS-CON SC Series.

120 µF/35V

Sanyo MV-GX Series.

120 µF/35V

Nichicon PL Series.

120 µF/35V

Panasonic HFQ Series.

Other manufacturers or other types of capacitors may also be

used, provided the capacitor specifications (especially the 100

kHz ESR) closely match the characteristics of the capacitors

listed in the output capacitor table. Refer to the capacitor

manufacturers’ data sheet for this information.

4. Catch Diode Selection (D1)

A. In normal operation, the average current of the catch diode is

the load current times the catch diode duty cycle, 1-D (D is the

switch duty cycle, which is approximately V

OUT/VIN). The largest

value of the catch diode average current occurs at the maximum

input voltage (minimum D). For normal operation, the catch

diode current rating must be at least 1.3 times greater than its

maximum average current. However, if the power supply design

must withstand a continuous output short, the diode should have

a current rating greater than the maximum current limit of the

LM2672. The most stressful condition for this diode is a shorted

output condition.

4. Catch Diode Selection (D1)

A. Refer to the table shown in Figure 12. Schottky diodes

provide the best performance, and in this example a 1A, 40V

Schottky diode would be a good choice. If the circuit must

withstand a continuous shorted output, a higher current (at least

2.2A) Schottky diode is recommended.

B. The reverse voltage rating of the diode should be at least

1.25 times the maximum input voltage.

C. Because of their fast switching speed and low forward

voltage drop, Schottky diodes provide the best performance and

efficiency. The Schottky diode must be located close to the

LM2672 using short leads and short printed circuit traces.

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